This KB will show you how to disable Offline OWA on Exchange 2013.
This feature will prevent users from enablaing Offline OWA which is available by defualt.
Once disabling the options users will not be able to turn Offline OWA on.
To disable the feature open EAC -> Permissions -> Outlook Web App Policies and edit the defualt policy
In the policy page go to offline access and select Never
Set the option to Never
You can also use Exchange Shell to do it using the following cmdlet:
Set-OwaMailboxPolicy –AllowOfflineOn NoComputers